The present invention relates to improving the permeability characteristics of a substerranean formation, and, in particular, to a polymeric material which can provide a stable gel at severe reservoir conditions.
Subterranean oil producing formations generally have variable permeability. In zones where permeability stratification exists, the waterflooding or other displacing fluid used in producing oil sweeps fastest through the more permeable zones so that much of the oil in the less permeable zones must be produced at high water (fluid) to oil ratios.
Polymer gels have been used to control the reservoir permeability profile. Effective gels require good strength to withstand long-term flooding. Furthermore, these polymer gels should be able to withstand the severe conditions present in substerranean reservoirs during enhanced oil recovery. Specifically, the gels should be able to withstand the presence of high salt concentrations especially since most enhanced oil recovery processes include the use of field brines, and should be able to withstand high temperatures without breaking down. In addition to these characteristics, it is preferable that the polymeric gel be easy to prepare for injection into the formation through an introducing well.
Polymers and gels useful for permeability control, as described in the prior art, are generally homopolymers and random copolymers. These polymers generally form homogeneous solutions and uniform bulk gels when crosslinked.
It has been known to use polyacrylamide and xanthan gums which can be crosslinked by a metal in order to improve profile control in waterflooded wells. Polyacrylamides, however, display instability in the presence of high brine concentration at temperatures over 70.degree. C. Xanthan gums, on the other hand, are very brine tolerant but display thermal instability, even at temperatures below 60.degree. C.
In view, therefore, of the severe reservoir conditions which include both high brine concentrations and elevated temperatures, there is a need for brine tolerant, thermally-stable materials suitable for high temperature wells and steam flooded wells.
It has been known, for example, to use polyvinylalcohol (PVA) metal-crosslinked gels, as described in U.S. Pat. No. 3,762,476 to Gall, in order to correct subterranean formation permeability. See also U.S. Pat. No. 4,039,029 to Gall and U.S. Pat. No. 4,018,286 to Gall, et al. Unfortunately, polyvinylalcohol has a drawback in that it has very poor cold water solubility, and has to be dispersed in cold water as a suspension followed by heating to a temperature of for example, 90.degree.-95.degree. C. for dissolution. Furthermore, polyvinylalcohol also has the limitation that the choice of possible crosslinking metal components is limited, Titanium (IV) probably being the only practical choice. Other commonly used metals such as aluminum, chromium, and zirconium are not effective to crosslink polyvinylalcohol. Furthermore, polyvinylalcohol gels crosslinked with titanium are not brine stable at high temperatures. In particular, syneresis can occur, i.e., the contraction of the gel on standing with exudation of a liquid, which detracts from predictable, stable profile control in the formation.
Polyvinylalcohol can also be crosslinked covalently, however, with polyaldehydes as described in Patent Application by Marrocco, et al., GB No. 2,145,420 A. The gelation described in the Marrocco, et al. disclosure, however, requires an acidic environment, creating a need for controlling the pH of the reservoir to facilitate the process. Needless to say, control of the reservoir environment at a particular pH can be a very difficult task. Regardless of the alternative crosslinking process described by Marrocco, et al., the polyvinylalcohol solution preparation problems remain and the gel stability in strong brine concentration is uncertain.
It is, therefore, an object of the present invention to provide a polymer gel for injection in a subterranean reservoir which has good thermal stability at high brine concentration.
It is a further object of the present invention to provide a highly stable polymeric gel composition which can be crosslinked with a variety of crosslinking metal ions.
It is still a further object of the present invention to provide a crosslinking gel composition which is easily prepared in a water solution without heating.
Yet another object of the present invention is to provide a polymeric gel composition which does not require control of the pH condition of the subterranean reservoir in order to effect crosslinking.
A further object of the present invention is to provide a profile control polymeric gel composition which is essentially nonhydrolizable.